Disk Substructures At High Angular Resolution Project Research Articles

Underestimation of the dust mass in protoplanetary disks: Effects of disk structure and dust properties

The total number of dust grains in protoplanetary disks is one of the key properties that characterizes the potential for planet formation. With (sub-)millimeter flux measurements, literature studies usually derive the dust mass using an analytic formula under the assumption of optically thin emission, which may lead to a substantial underestimation. In this work, we conduct a parameter study with the goal of investigating the effects of disk structure and dust properties on the underestimation through self-consistent radiative transfer models. Different dust models, scattering modes, and approaches for dust settling are considered and compared. The influences of disk substructures, such as rings and crescents, on the mass derivation are investigated as well. The results indicate that the traditional analytic method can underestimate the mass by a factor of a few to several hundreds, depending on the optical depth along the line of sight set mainly by the true dust mass, disk size, and inclination. As an application, we performed a detailed radiative transfer modeling of the spectral energy distribution of DoAr 33, one of the disks observed by the Disk Substructures at High Angular Resolution Project (DSHARP). When the DSHARP dust opacities are adopted, the most probable dust mass returned from the Bayesian analysis is roughly seven times higher than the value given by the analytic calculation. Our study demonstrates that estimating disk dust masses from radiative transfer modeling is one solution to alleviate the problem of insufficient mass for planet formation that was raised in the era of the Atacama Large Millimeter/submillimeter Array.

Limits on Millimeter Continuum Emission from Circumplanetary Material in the DSHARP Disks

Abstract We present a detailed analysis for a subset of the high-resolution (∼35 mas, or 5 au) ALMA observations from the Disk Substructures at High Angular Resolution Project (DSHARP) to search for faint 1.3 mm continuum emission associated with dusty circumplanetary material located within the narrow annuli of depleted emission (gaps) in circumstellar disks. This search used the Jennings et al. frank modeling methodology to mitigate contamination from the local disk emission and then deployed a suite of injection–recovery experiments to statistically characterize point-like circumplanetary disks in residual images. While there are a few putative candidates in this sample, they have only marginal local signal-to-noise ratios and would require deeper measurements to confirm. Associating a 50% recovery fraction with an upper limit, we find that these data are sensitive to circumplanetary disks with flux densities ≳50–70 μJy in most cases. There are a few examples where those limits are inflated (≳110 μJy) owing to lingering nonaxisymmetric structures in their host circumstellar disks, most notably for a newly identified faint spiral in the HD 143006 disk. For standard assumptions, this analysis suggests that these data should be sensitive to circumplanetary disks with dust masses ≳ 0.001 – 0.2 M ⊕ . While those bounds are comparable to some theoretical expectations for young giant planets, we discuss how plausible system properties (e.g., relatively low host planet masses or the efficient radial drift of solids) could require much deeper observations to achieve robust detections.

Estimate on Dust Scale Height from the ALMA Dust Continuum Image of the HD 163296 Protoplanetary Disk

Abstract We aim at estimating the dust scale height of protoplanetary disks from millimeter continuum observations. First, we present a general expression of intensity of a ring in a protoplanetary disk and show that we can constrain the dust scale height by the azimuthal intensity variation. Then, we apply the presented methodology to the two distinct rings at 68 au and at 100 au of the protoplanetary disk around HD 163296. We constrain the dust scale height by comparing the high-resolution millimeter dust continuum image obtained in the Disk Substructures at High Angular Resolution Project (DSHARP) with radiative transfer simulations using RADMC-3D. We find that h d/h g > 0.84 at the inner ring and h d/h g < 0.11 at the outer ring with 3σ uncertainties, where h d is the dust scale height and h g is the gas scale height. This indicates that the dust is flared at the inner ring and settled at the outer ring. We further constrain the ratio of the turbulence parameter α to the gas-to-dust-coupling parameter St from the derived dust scale height; α/St > 2.4 at the inner ring, and α/St < 1.1 × 10 − 2 at the outer ring. This result shows that the turbulence is stronger or the dust is smaller at the inner ring than at the outer ring.

One Solution to the Mass Budget Problem for Planet Formation: Optically Thick Disks with Dust Scattering

Abstract Atacama Large Millimeter Array (ALMA) surveys have suggested that the dust in Class II disks may not be enough to explain the averaged solid mass in exoplanets, under the assumption that the mm disk continuum emission is optically thin. This optically thin assumption seems to be supported by recent Disk Substructures at High Angular Resolution Project (DSHARP) observations where the measured optical depths are mostly less than one. However, we point out that dust scattering can considerably reduce the emission from an optically thick region. If that scattering is ignored, an optically thick disk with scattering can be misidentified as an optically thin disk. Dust scattering in more inclined disks can reduce the intensity even further, making the disk look even fainter. The measured optical depth of ∼0.6 in several DSHARP disks can be naturally explained by optically thick dust with an albedo of ∼0.9 at 1.25 mm. Using the DSHARP opacity, this albedo corresponds to a dust population with the maximum grain size (s max) of 0.1–1 mm. For optically thick scattering disks, the measured spectral index α can be either larger or smaller than 2 depending on whether the dust albedo increases or decreases with wavelength. We describe how this optically thick scattering scenario could explain the observed scaling between submm continuum sizes and luminosities, and might help ease the tension between the dust size constraints from polarization and dust continuum measurements. We suggest that a significant amount of disk mass can be hidden from ALMA observations and longer wavelength observations (e.g., Very Large Array or Square Kilometre Array) are desired to probe the dust mass in disks.

The Disk Substructures at High Angular Resolution Project (DSHARP). V. Interpreting ALMA Maps of Protoplanetary Disks in Terms of a Dust Model

Abstract The Disk Substructures at High Angular Resolution Project (DSHARP) is the largest homogeneous high-resolution (∼0.″035, or ∼5 au) disk continuum imaging survey with the Atacama Large Millimeter/submillimeter Array (ALMA) so far. In the coming years, many more disks will be mapped with ALMA at similar resolution. Interpreting the results in terms of the properties and quantities of the emitting dusty material is, however, a very non-trivial task. This is in part due to the uncertainty in the dust opacities, an uncertainty that is not likely to be resolved any time soon. It is also partly due to the fact that, as the DSHARP survey has shown, these disk often contain regions of intermediate to high optical depth, even at millimeter wavelengths and at relatively large radius in the disk. This makes the interpretation challenging, in particular if the grains are large and have a large albedo. On the other hand, the highly structured features seen in the DSHARP survey, of which strong indications were already seen in earlier observations, provide a unique opportunity to study the dust growth and dynamics. To provide continuity within the DSHARP project, its follow-up projects, and projects by other teams interested in these data, we present here the methods and opacity choices used within the DSHARP collaboration to link the measured intensity I ν to dust surface density Σ d .

The Disk Substructures at High Angular Resolution Project (DSHARP). I. Motivation, Sample, Calibration, and Overview

Abstract We introduce the Disk Substructures at High Angular Resolution Project (DSHARP), one of the initial Large Programs conducted with the Atacama Large Millimeter/submillimeter Array (ALMA). The primary goal of DSHARP is to find and characterize substructures in the spatial distributions of solid particles for a sample of 20 nearby protoplanetary disks, using very high resolution (∼0.″035, or 5 au, FWHM) observations of their 240 GHz (1.25 mm) continuum emission. These data provide a first homogeneous look at the small-scale features in disks that are directly relevant to the planet formation process, quantifying their prevalence, morphologies, spatial scales, spacings, symmetry, and amplitudes, for targets with a variety of disk and stellar host properties. We find that these substructures are ubiquitous in this sample of large, bright disks. They are most frequently manifested as concentric, narrow emission rings and depleted gaps, although large-scale spiral patterns and small arc-shaped azimuthal asymmetries are also present in some cases. These substructures are found at a wide range of disk radii (from a few astronomical units to more than 100 au), are usually compact (≲10 au), and show a wide range of amplitudes (brightness contrasts). Here we discuss the motivation for the project, describe the survey design and the sample properties, detail the observations and data calibration, highlight some basic results, and provide a general overview of the key conclusions that are presented in more detail in a series of accompanying articles. The DSHARP data—including visibilities, images, calibration scripts, and more—are released for community use at https://almascience.org/alma-data/lp/DSHARP.

The Disk Substructures at High Angular Resolution Project (DSHARP). IV. Characterizing Substructures and Interactions in Disks around Multiple Star Systems

Abstract To characterize the substructures induced in protoplanetary disks by the interaction between stars in multiple systems, we study the 1.25 mm continuum and the 12CO(J = 2–1) spectral line emission of the triple systems HT Lup and AS 205, at scales of ≈5 au, as part of the “Disk Substructures at High Angular Resolution Project” (DSHARP). In the continuum emission, we find two symmetric spiral arms in the disk around AS 205 N, with a pitch angle of 14°, while the southern component AS 205 S, itself a spectroscopic binary, is surrounded by a compact inner disk and a bright ring at a radius of 34 au. The 12CO line exhibits clear signatures of tidal interactions, with spiral arms, extended arc-like emission, and high velocity gas, possible evidence of a recent close encounter between the disks in the AS 205 system, as these features are predicted by hydrodynamic simulations of flyby encounters. In the HT Lup system, we detect continuum emission from all three components. The primary disk, HT Lup A, also shows a two-armed symmetric spiral structure with a pitch angle of 4°, while HT Lup B and C, located at 25 and 434 au in projected separation from HT Lup A, are barely resolved with ∼5 and ∼10 au in diameter, respectively. The gas kinematics for the closest pair indicates a different sense of rotation for each disk, which could be explained by either a counter rotation of the two disks in different, close to parallel, planes, or by a projection effect of these disks with a close to 90° misalignment between them.

The Disk Substructures at High Angular Resolution Project (DSHARP). VII. The Planet–Disk Interactions Interpretation

Abstract The Disk Substructures at High Angular Resolution Project (DSHARP) provides a large sample of protoplanetary disks with substructures that could be induced by young forming planets. To explore the properties of planets that may be responsible for these substructures, we systematically carry out a grid of 2D hydrodynamical simulations, including both gas and dust components. We present the resulting gas structures, including the relationship between the planet mass, as well as (1) the gaseous gap depth/width and (2) the sub/super-Keplerian motion across the gap. We then compute dust continuum intensity maps at the frequency of the DSHARP observations. We provide the relationship between the planet mass, as well as (1) the depth/width of the gaps at millimeter intensity maps, (2) the gap edge ellipticity and asymmetry, and (3) the position of secondary gaps induced by the planet. With these relationships, we lay out the procedure to constrain the planet mass using gap properties, and study the potential planets in the DSHARP disks. We highlight the excellent agreement between observations and simulations for AS 209 and the detectability of the young solar system analog. Finally, under the assumption that the detected gaps are induced by young planets, we characterize the young planet population in the planet mass–semimajor axis diagram. We find that the occurrence rate for >5 M J planets beyond 5–10 au is consistent with direct imaging constraints. Disk substructures allow us to probe a wide-orbit planet population (Neptune to Jupiter mass planets beyond 10 au) that is not accessible to other planet searching techniques.

The Disk Substructures at High Angular Resolution Project (DSHARP). X. Multiple Rings, a Misaligned Inner Disk, and a Bright Arc in the Disk around the T Tauri star HD 143006

Abstract We present a detailed analysis of new Atacama Large Millimeter/submillimeter Array (ALMA) observations of the disk around the T-Tauri star HD 143006, which at 46 mas (7.6 au) resolution reveals new substructures in the 1.25 mm continuum emission. The disk resolves into a series of concentric rings and gaps, together with a bright arc exterior to the rings that resembles hydrodynamical simulations of a vortex and a bridge-like feature connecting the two innermost rings. Although our 12CO observations at similar spatial resolution do not show obvious substructure, they reveal an inner disk depleted of CO emission. From the continuum emission and the CO velocity field we find that the innermost ring has a higher inclination than the outermost rings and the arc. This is evidence for either a small (∼8°) or moderate (∼41°) misalignment between the inner and outer disk, depending on the specific orientation of the near/far sides of the inner/outer disk. We compare the observed substructures in the ALMA observations with recent scattered-light data of this object from the Very Large Telescope/Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE). In particular, the location of narrow shadow lanes in the SPHERE image, combined with pressure-scale height estimates, favor a large misalignment of about 41°. We discuss our findings in the context of a dust-trapping vortex, planet-carved gaps, and a misaligned inner disk due to the presence of an inclined companion to HD 143006.